The present invention relates to a method for the manufacture of crystals of sodium carbonate monohydrate from carbonated aqueous solutions of sodium hydroxide.
More particularly, the present invention relates to crystallization of sodium carbonate monohydrate by carbonation of caustic liquors such as those obtained by electrolysis of sodium chloride brines.
In a known method of manufacturing sodium carbonate monohydrate, described in Belgian Pat. No. 597,474 filed on Nov. 24th, 1960, in the name of Imperial Chemical Industries Limited, sodium carbonate monohydrate is crystallized by mixing an aqueous solution containing at least 56% by weight of sodium hydroxide with a carbonated solution, the crystals obtained and a mother liquor are separated, and the mother liquor is treated with a gas containing CO.sub.2 in order to form the aforesaid carbonated solution.
In this known method, the aqueous solution of sodium hydroxide may result from the decomposition of a sodium amalgam obtained by the electrolysis of a sodium chloride brine in a cell with a mercury cathode.
The sodium carbonate monohydrate obtained by this method is intended mainly for the production of dense sodium carbonate, to be employed, for example, in the steel industry and the glass industry.
In practice, this known method does, however, have the disadvantage of producing poorly crystallized sodium carbonate monohydrate. The anhydrous sodium carbonate obtained by calcining these sodium carbonate monohydrate crystals has an apparent specific gravity that is too low for efficient use in steel and glass manufacture.
In U.S. Pat. No. 3,628,919 to Beauchamp, there is described a process for crystallizing sodium carbonate (possibly monohydrated) starting from aqueous solutions containing organic materials. It aims more particularly at furnishing a process which avoids the unfavorable action of these organic materials on the sodium carbonate crystals. According to the Beauchamp process, sodium carbonate is crystallized onto seed crystals obtained under good crystallization conditions. The choice of the source of seed crystals is not however critical. Further, the Beauchamp patent is not concerned with the use of carbonated aqueous solutions of sodium hydroxide.